The disclosure generally relates to aqueous coating compositions for leather. The finishing of leather imparts to the leather articles their fashion aspect and their suitability for use. Finishing is understood as meaning the application of binders, dyes, pigments, waxes, handle compositions and further auxiliaries, by customary application techniques, such as spraying, printing, pouring, knife-coating, and application with a plush pad, to the tanned hide. Application can be done in one coat but is generally carried out in a plurality of coats, with further process steps, such as intermediate drying, plating, embossing, milling, being customary. After each application of a finish coat, the leathers are generally stacked. This is possible only if the freshly applied finish coat does not stick to the adjacent leather article in the stack after drying. The conditions for drying an aqueous finish coating on leather are limited and cannot far exceed the shrinkage temperature of the leather (about 120° C. for chrome-tanned leather and about 80° C. for organic tanned leather). Also, the drying time must be short. Owing to these limitations, film-forming polymer dispersions which, after drying, provide a non-tacky finish coat having good mechanical strength are used in the finishing of leather with aqueous coating compositions.
Water-emulsifiable hexamethylene diisocyanate-based crosslinkers have traditionally been used in leather finishing applications, but they suffer from a short activated pot-life when blended with an aqueous coating composition prior to application. Additionally, stored activated coating compositions containing hexamethylene diisocyanate-based crosslinkers usually congeal or are no longer of use in a relatively short time of approximately 6 hours. Thus, polyisocyanate-based leather coating compositions with improved storage stability, and one or more of improved mechanical properties, minimal embrittlement, and low fresh tack, are desirable.